The present invention relates to moisture stable temperature indicating compositions that produce a visible and permanent color change upon exposure to a predetermined heat history. The present invention also relates to indicator materials containing coatings of the temperature indicating compositions on at least one surface of a suitable support. In particular, the present invention relates to temperature indicating compositions and coatings thereof that do not fully undergo a color change until the coating has been exposed to saturated steam at a temperature of 250.degree. F. for more than 3 minutes. They do, however, signal fully after 20 minutes in the presence of saturated steam at 250.degree. F. Products presently available to meet this need suffer from several deficiencies. Many are formulated with metallic salts such as carbonates of copper or lead. Color development comes from the reaction of a sulfidic component, present as sulfur, thiosulfate, or a sulfur containing organic compound (U.S. Pat. Nos. 3,471,222 and 3,523,011) to form a black metallic sulfide.
The resulting deficiencies include:
(1) a limitation in the final signal color because of the blackness of the metallic sulfide; PA1 (2) a poor resistance to elevated, but not unusual, levels of temperature and humidity during transport and storage. Copper-based inks are particularly susceptible to premature signaling under these "tropical" conditions; and PA1 (3) materials printed with lead containing inks are increasingly unsuitable for disposal in land fills, or by incineration, in countries whose environmental sensitivities and current legislation are significantly more restrictive than in earlier times.
The products that will be further described below overcome these deficiencies by (1) being available in a range of final signal colors, (2) being resistant to signaling during "tropical" storage conditions and (3) being completely free of environmentally regulated compounds of lead or copper.
The color-changing reaction between an electron donating compound such as a prodye or chromogen and an electron accepting compound, typically a phenolic compound, has been widely used in the field of duplication and printing processes for the preparation of heat sensitive coating compositions, for the production of heat sensitive paper for thermal printing devices (see e.g., U.S. Pat. No. 4,289,535, and the patents cited therein, as well as U.S. Pat. Nos. 4,287,264, 4,228,222 and 3,539,375). This color-changing reaction has also been widely used in the fields of medicine and food processing for the production of heat sensitive compositions suitable for coating on the surface of supports to form sterilization indicators (see, e.g., U.S. Pat. No. 5,087,659, 4,155,895 and 4,179,397).
Coatings of such heat-sensitive compositions are intended to show a distinct difference in color among unprocessed and completely processed packages or foodstuffs. This provides a positive and visible indication that the sterilization process has in fact been carried out.
As discussed in the above-cited U.S. Pat. Nos. 4,287,264 and 4,289,535, state-of-the-art compositions combining an electron donating prodye or chromogen compound with an electron accepting phenolic compound suffer from a tendency to undergo premature color change which is attributable to a lack of environmental stability, particularly to heat and moisture (i.e., humidity). Unthermostated shipping and warehouse storage conditions, as well as tropical climates, are well-known as causes of premature color-change.
While premature color-change is an inconvenience for the thermal printing industry because it ruins shipments of heat-sensitive paper, the phenomena is a critical problem for the medical profession and food processing industry because the premature color change creates a false-positive indication that medical and surgical goods have been processed or sterilized. There remains a need for temperature indicator compositions that undergo a visible and permanent color change under sterilization conditions, yet at the same time do not undergo premature color change upon exposure to elevated shipping and storage temperatures or the heat and humidity of tropical climates.